What are the health impacts of using cannabis?

The Justice Minister Andrew Little hopes legalising recreational cannabis will reduce the harm it can cause and lower the overall use of the drug over time. People would have to be 20 to legally buy and use cannabis - and they'd be able to smoke it only at home or at special bars. It would also only be sold only at licensed stores - at regulated levels of potency. Doctor Joseph Boden investigated the use of cannabis by one thousand participants in a longitudinal study which started in the 1970s.